This invention relates to electrically and mechanically joining a relatively flexible circuit-bearing element to a relatively inflexible circuit-bearing element, each having multiple arrays of closely spaced electrically conductive traces.
Electrical and electronic devices often formed by joining together separate elements, each containing electronic circuitry. The separate elements are electrically connected by providing each element with electrical contact pads or traces, aligning the contact pads or traces on one element with the corresponding contact pads on the other element, and then bonding the elements together in a manner that provides electrical conductivity between the aligned contact pads or traces.
One combination of circuit elements is a rigid or inflexible substrate, such as an electronic circuit board, bearing electronic circuitry, joined to a circuit-bearing flexible substrate, such as a flexible circuit film. The circuitry on each substrate contains contact pads on the surface of the substrate. The flexible circuit film and the rigid electronic circuit board are joined by a joining layer between them. The joining layer is formed of a material, such as anisotropic conductive film, that, under compression, is electrically conductive in the direction of compression, but remains substantially insulating in the directions orthogonal to the direction of compression. Bonding the anisotropy conductive film between the rigid electronic circuit board and the flexible circuit film is performed by arranging the electronic circuit board, the anisotropy conductive film, and the flexible circuit film on a tabular fixture, and pressing a heated bond shoe onto the assembled elements. An exemplary bonding apparatus is available from DCI, Inc. of Lenexa, Kans., as Model No. 1093C Heat Seal Press.
An existing bonding apparatus uses a single heated bond shoe that is typically up to approximately 2-3 inches (5.0-7.5 cm) in length, and 0.3 inches in width to bond circuit elements having an array of contact pads up to the length of the bond shoe. However, some electronic devices contain circuit elements having arrays of contact pads in which the dimensions of the array exceed this bond shoe length. For example, a large printhead for an ink jet printer may have contact pads arranged in arrays that are up to 8-9 inches (20-23 cm) in length. In some instances, the arrays of contact pads may be quite dense (150 contact pads per inch) or the contact pads may be arranged in multiple rows along this length. Each of these circumstances requires that the bonding technique be highly accurate, to ensure the corresponding contact pads on the circuit elements are accurately connected electrically, and that non-corresponding contact pads are not improperly connected.
For joining large arrays of contact pads, two techniques have been available. One technique is to use a bond shoe that is as long as the array of contact pads. This technique requires that the face of the bond shoe be exceptionally flat, so that the bond shoe applies pressure evenly along the entire array of contact pads. The nature of the anisotropic conductive film is such that sections subjected to different forces of compression have different electrical conductivity. Variation in the face of the bond shoe may cause variations in the performance of the resulting electrical connections. The other technique is to use a standard sized bond shoe to bond one section of the array, then relocate the assembly of circuit elements on the fixture of the bonding apparatus, and use the same bond shoe to bond an adjacent section of the array. This technique requires accurately placing and the circuit bearing elements multiple times on the fixture during the bonding process. Each relocation of the circuit bearing elements creates a risk of damaging the elements, or of inaccurately aligning the connections between the elements. In addition, the repeated relocations of the circuit elements slow production processes.